The present invention relates to document security and in particular to a security substrate for documents of value such as for example, banknotes, credit cards, identification cards and the like.
Documents of value such as identification cards, credit cards, banknotes, etc. are in many cases formed of polymer-based laminates. For example, U.S. Pat. No. 5,935,696 to Benoit et al. discloses a laminated multilayer film substrate having high-density polyethylene layers on each side of an oriented polypropylene layer. The multilayer film substrate exhibits good embossability, dead-fold characteristics and other properties making it suitable for the production of banknotes and other security documents.
Marking documents of value is common practice to deter forgery and allow counterfeit cards and banknotes to be readily detected. For example, U.S. Pat. No. 3,858,977 to Baird et al. discloses an optical interference filter having an optical interference layer with a known characteristic of spectral reflectance and a different known characteristic of spectral transmittance, both of which vary with the angle of incident light on the interference filter. The interference filter is disposed on the banknote substrate over a coloured portion thereof designed to absorb some of the light transmitted by the interference filter so that the interference filer exhibits a colour change with a change in the angle of incident light.
In order to inhibit forgery and counterfeiting, it is desired that security features used to mark documents of value be difficult to copy and highly secured against alteration. Accordingly, improved substrates for documents of value are desired.
It is therefore an object of the present invention to provide a novel security substrate for a document of value and method of forming the same.
According to one aspect of the present invention there is provided a security substrate for a document of value comprising:
a laminate including a generally transparent core layer and a generally transparent outer layer on at least one side of said core layer; and
at least one interference filter embedded within said laminate, said interference layer including a highly reflective opaque layer on a surface of said core layer and an optically variable thin film multilayer overlying at least a portion of said opaque layer, said interference filter being covered by said outer layer.
In a preferred embodiment, the opaque metal layer is patterned on the core layer at spaced locations. The opaque metal layer can be patterned to form continuous stripes or discrete elements that may or may not be intricately patterned.
Preferably, the optically variable thin film multilayer includes one or more dielectric film layers of low refractive index and a semi-transparent, medium reflective layer.
Preferably, generally transparent outer layers are provided on opposite sides of the core layer. Opacifying coatings can be applied to the outer layers and have windows formed therein that are in registration with the interference filter.
According to another aspect of the present invention there is provided a security substrate for a document of value comprising:
a laminate including a generally transparent balanced biaxially oriented core layer, said core layer being oriented in at least a first direction at an orientation ratio of at least 4:1 and oriented in a second direction substantially normal to the first direction at an orientation ratio of at least 6:1; generally transparent imbalanced biaxially oriented outer layers on opposite sides of said core layer, said outer layers being oriented in at least a first direction to a degree which is at least three times less than the degree of orientation present in a second direction substantially normal to the first direction; and a laminating adhesive resin disposed between the outer layers and the core layer to secure the outer layers to the core layer so that the first directions of orientation of the outer layers are substantially aligned;
at least one optically variable device embedded within the laminate, the at least one optically variable device including a highly reflective opaque layer on a surface of said core layer and an optically variable thin film multilayer overlying at least a portion of the opaque layer, said at least one optically variable device being covered by one of said outer layers; and
opacifying coatings on said outer layers, at least one window being formed in at least one of said opacifying coatings that is in registration with said at least one optically variable device.
According to another aspect of the present invention there is provided a method of forming a security substrate for a document of value comprising the steps of:
depositing a highly reflective opaque layer on a surface of a generally transparent core layer;
depositing an optically variable thin film multilayer on said core layer to overlie at least a portion of said opaque layer; and
overlying at least one side of said core layer with a generally transparent outer layer to cover said opaque layer and optically variable thin film multilayer thereby to embed an optically variable device within said security substrate
The present invention provides advantages in that the optically variable interference filter is embedded in the security substrate protecting it from mechanical and chemical attack as well as wear and tear. As a result, the security substrate exhibits high longevity and is highly secured against alternation. Also, since the optically variable interference filter exhibits a sharp color shifting characteristic with changes in the angle of view, the security substrate is virtually impossible to counterfeit using digital methods, laser scanning or photocopying. Furthermore, the security substrate exhibits high tensile strength in the oriented and unoriented directions, good folding and crumple resistance.